Field of the Invention
Frequency converters for three-phase motors nowadays operate almost without exception using microcontrollers which take over the whole task of controlling and generating the drive signals for the stator coils of the motor. The drive signals here are pulse-width modulated (PWM), so that the desired position of the rotary field can be adjusted by the pulse duration of the signals. This method is also to be used for motors with high rotary speeds and at high drive frequencies. The switching frequency is critical here in respect of the desired sinusoidal form of the motor drive signals.
In order to ensure optimum drive to the motor, offset clocking is chosen as the drive scheme. The microcontroller here modulates both edges of the PWM signals. The motor thus receives an appropriate voltage pulse at each edge. Inside the controller this is preferably done by a timer that counts upwards from zero to a maximum for the first edge, and counts downwards to zero for the second edge. The contents of three compare registers for the three phases are compared with the counter status. The raw signals for the end-stage transistors are generated from the counter direction and the compare-match signals.
Since at high rotary speeds the motor has already significantly turned within the half-period of the PWM signal, the compare values are preferably calculated afresh for each edge. For example, at 100,000 rpm, with two pole pairs and a switching frequency of 25 kHz, the field of the motor turns by 24° between one switching edge and the next switching edge. Thus at a switching frequency of 25 kHz, a time of only 20 μs is available. This means that it is necessary to use a microcontroller with a very high computing power.
Since the necessary computing power is not usually available with the microprocessors typically used for such applications, compromises must be made. Frequently, a first step is to omit calculation of both edges. This halves the necessary computing power. Parts of the field-oriented control are then calculated at an even lower rate, which is in fact fully adequate in the case, for example, of a speed controller, due to its low bandwidth. This does, however, lead to an unsatisfactory lack of precision.